A well known problem with conventional Passive Infra Red (PIR) occupancy circuits that use a relay output in a two wire system (i.e., no neutral) is that when the relay contact is closed, there is no power available to drive the control circuitry since the relay contacts short circuit the control circuitry. This problem is not exclusive to PIR occupancy circuits. In fact, any generic two wire electrical control device that switches power across a load when energized may display a similar problem, i.e., when the switched contact is in a low impedance state (the relay contacts are closed), the voltage across the device drops from a level approximate to that of the AC line voltage to almost zero. Thus, during the time the control device is on (energized), no power is available to drive the switching control circuitry.
One solution known in the art utilizes a technique whereby a small amount of current is purposely leaked to ground to drive control circuitry when power is switched across the load. The switching control circuitry, if designed so as to require a small amount of current to keep it operational (compared to the load circuitry), can derive the power it needs for operation from this ground leakage current. Underwriters Laboratory (UL) allows electrical devices 0.5 ma of leakage current wherefore such ground leakage current operation can be arranged. However, the 0.5 ma leakage current limitation makes designing using this technique difficult to implement.
For example, U.S. Pat. No. 4,713,598 to Smith discloses a power supply circuit for generating power from a switched AC source. The circuit includes a current transformer arranged in series with a controlled main conduction path (contact) of a relay switch disposed in the AC source/load main line. A series combination of a capacitor and a secondary winding of the transformer shunt the primary winding/relay contact series combination. When the relay switch is conductive, a comparatively small AC voltage appears across a secondary of the transformer which is rectified with a rectifying diode electrically connected to power an amplifier. A capacitor connected in shunt with the amplifier filters the DC generated by the diode. The amplifier is driven by a detection circuit (e.g., a passive infrared detector) which drives the relay switch. When the contact is in a non-conducting state (i.e., a high impedance state), no current flows in the transformer's primary. However, because little voltage is dropped across the load, almost the full potential of the AC source appears across the blocking capacitor/transformer secondary series combination. This open circuit potential is used to power the circuitry when the relay is non-conducting, i.e., ground leakage current.
U.S. Pat. No. 4,336,464 to Weber discloses a two-terminal timed electric switch for series connection with one side of a power-carrying AC circuit. An AC line terminal is electrically connected in series through a primary of a current transformer and a contact of a relay switch to a load. The load's other terminal is connected to the AC neutral. While the load is energized, the transformer's secondary provides power to a timer circuit. The circuit is energized when a momentary action start ("on") switch is temporarily closed (pressed) whereby the power is generated in the secondary for closing the relay contact. This momentary contact switch must be actuated before the Weber circuitry can be actuated. For example, were the timer circuit to be a PIR occupancy circuit, operation of the PIR circuitry would first require momentary closure of the momentary switch.
It would be beneficial, therefore, to realize a device for use in two-wire detector or sensor circuit which utilizes an energy source for operating the sensor independent of load activation or ground leakage current. The energy source could be independent from current operation, or dependent thereon, e.g., a charge storage device. It would also be beneficial to have a device for use in a two-wire sensor or detector circuit wherein a current transformer is utilized to indirectly supply the sensor or charge storage device during a time at which said load is powered by said AC source thereby minimizing the storage requirements of the charge storage device.